It has long been the conventional wisdom in Scotland that Scottish voters are essentially constitutional opportunists. They may support the SNP in Holyrood elections, to "fight Scotland's corner", but in the end they will always vote to stay in the UK. Historically, support for independence has always lagged behind support for the Scottish National party. But perhaps for not much longer.

The TNS System Three poll in the Sunday Herald is the first to show a slim majority for independence along the lines proposed by the SNP. TNS asked the very question that the SNP intend to put in their constitutional referendum pencilled in for 2010: "The Scottish government should negotiate a settlement with the government of the United Kingdom so that Scotland becomes an independent state: Yes or No." Of those asked 41% said yes and 40% said no. Only last August, the same poll registered a 15% lead for the noes.

This follows a poll last week by Scottish Opinion which also showed support for independence surging over the 40% mark on the same question. We can't say conclusively that Scots have made up their minds to leave the UK, but there is no doubt that something is stirring in the undergrowth. It suggests that Alex Salmond may now be succeeding in his drive to convert support for his party into support for leaving the UK. The question is, why?

Now, obviously, the impressive performance of the SNP minority administration in Holyrood over the last year has had something to do with this. Salmond has delivered a masterclass in nationalist populism, scrapping university fees, abolishing prescription charges, freezing council tax. However, the really interesting question is why Gordon Brown, for all his immersion in Scottish politics, has had no answers to all this. Clearly, his mind has been elsewhere.

Most commentators believed that Scots would begin lose their affection for nationalism as soon as Gordon Brown became prime minister last autumn. After all, what better advert for the Union than to have one of the most respected Scottish politicians in Downing St. It showed that Scotland still mattered and that Scotland could have a voice at the very highest levels of UK government. Unfortunately, Brown has been an unmitigated disaster for Labour in Scotland, and not just because of his reputation as a ditherer.

He installed his protege Wendy Alexander - without an election - as leader of the Scottish party, and plunged Labour into its worst crisis since the resignation of Henry McLeish as first minister in 2001. Alexander was supposed to be the fresh new face of Labour, but she has emerged as a deluded opportunist who claims to be "socialist" while getting caught up in a row about illegal donations from property developers; who mounts campaigns against cuts in services even as her mentor, Gordon Brown, was cutting the funding to pay for them.

Brown's recent threats to cut off council tax benefits and to overrule the Scottish parliament's power to introduce local income tax were politically inept. This just allowed the SNP to paint Labour as neocolonial governors and defenders of the hated council tax. The increase in whisky duty in the budget was so unpopular that even Wendy Alexander's husband, the economist Professor Brian Ashcroft, condemned it.

Perhaps worst of all, the abolition of the 10p tax band has hit Scots hard. The working poor are a depressing reality of modern Scotland and around 500,000 low-paid employees are likely to lose out as a result of this tax increase. Worse, Brown has offended the very social democratic Scotland that Wendy Alexander has been trying to appeal to in her recent speeches in which she claimed - improbably - to be dedicated to "the redistribution of wealth".

With political incompetence on this scale it is hardly surprising that Salmond has had a good year in office. The sense of dynamism in the new Scottish executive has clearly impressed Scottish people and converted their latent nationalism into something more overtly political. Yet the stubborn fact remains that everything the SNP has done has been within the terms of the devolution settlement - so in a sense this has been a success for devolution, not independence. It took the SNP to discover just how radical the Scotland Act really is.

The SNP is now urging Scots to move to the next level - full political independence. They seem to be nibbling, but there is a studied vagueness about exactly what full-scale independence would actually mean. The SNP propose to remain within the EU, retain the Queen as head of state, and keep sterling as Scotland's currency, at least for the time being. This looks more like federalism, or perhaps confederalism than old-style 19th century nation-state nationalism. It could even be a form of "devolution max" that is being proposed by the Liberal Democrats.

The real problem for Labour could be that the Scots are losing their fear of independence because it no longer seems to mean border posts at Gretna Green. Some in the Labour party believe that Brown should halt the slide and call Alex Salmond's bluff by holding an early referendum on independence. This would force the SNP to define independence and force the Scots to choose. However, there is no sign that Gordon Brown is in any fit state right now to stage a constitutional referendum - especially since he has just refused to contemplate one on the EU treaty.

Brown's implosion couldn't have happened at a worst time for the union - just as nationalists have entered government in all three devolved administrations. This prime minister seems to be incapable of doing very much at all now that his cabinet is in revolt. Labour's opinion poll slide in the UK and Scotland could have even more serious consequences. If Brown loses his majority at the next general election in Westminster, there could be a protracted period of constitutional instability in Westminster.

If the Conservatives take over in Westminster, then Scottish MPs will likely be barred from voting on nominally English legislation under the "English votes for English laws" policies of David Cameron. The Barnett Formula will be scrapped and Scotland will likely assume responsibility for its own tax-raising. It could be that the fall of Gordon Brown will be the beginning of the end for the United Kingdom.